The polarization coil and tiltable
platform upon which it sits must not
contain any ferromagnetic material, since this would alter the natural
uniformity of the Earth’s field, resulting in varying precession frequencies
of the protons within the sample, and weakening the signal induced in the
sample coil. For the same reason, care must be taken that there are no
ferromagnetic materials under or near the polarization coil and platform, and
the polarization coil/sensor coil should be placed outside of buildings, when
in operation, since most buildings contain large amounts of ferromagnetic
materials in their infrastructure (steel I-beams, airducts, nails, etc).

One of our customers once asked whether placing the entire coil assembly
inside of a large nonferromagnetic metal box would eliminate magnetic
field noise. This is definitely NOT a good idea. Doing so would induce
eddy currents in the box when the polarization coil current is turned off, corrupting the
magnetic field in the region of the sample, and dashing all hope of getting a
signal in the sample coil. For this reason, you want to keep away from the
polarization coil any large metallic objects, whether ferromagnetic or not.

A current of approximately 10 amps is sent through the
polarizing coil to polarize the sample. The wire size used for the
polarizing coil must be large enough to handle this current. We use a 14
gauge, 600 volt THHN insulated solid copper wire for the polarizing coil, and
a 16 gauge, 300 volt flexible stranded lamp cord for the 50
ft (15 meters) long connection from the polarizing coil to the pulse controller. The slightly thinner lamp
cord is OK since it is not wound like the coil wire and doesn’t have much more
resistance, but if you can find a 14 gauge lamp cord, you can use that. Note
that this would slightly increase the current through the polarizing coil.

The pulse controller turns off the approximately 10
amps of current going through the polarizing coil in about 300
milliseconds. This quick turnoff induces a large voltage of about 270 volts.
Therefore the voltage rating of the wire used in constructing the polarizing
coil is important. Note that we use a 600 volt wire for
the polarizing coil itself, and a 300 volt wire for connection to the coil. Of
course you can use a higher voltage wire than this. Keep in mind that the
voltage rating of a wire is solely dependent on the wire’s insulation, not the
wire itself.

The purpose of the polarizing coil is to produce a
magnetic field much greater than that of the Earth’s, so that the majority of
the population of protons in the sample are precessing about the field of the
coil and not the Earth’s field. For this reason, the field produced by the
polarizing coil need not be especially uniform, as long as it is large
compared to the Earth’s field. In fact this property inspired the name
Magnum since it is a brute force method of manipulating the spins of
nuclei.

The symmetry axis (the axis around which the coil is wound) of the
polarization coil should be oriented so that it is
perpendicular to the Earth’s magnetic field. In
the northern hemisphere, the magnetic field enters
the ground at an angle called the magnetic inclination. The polarization coil must therefore
be tilted from the horizontal by an amount equal to the magnetic inclination
angle. So to properly align the coil, orient the platform on which the coil is
mounted (using a standard magnetic compass ), such that
the axis about which the coil tilts, is perpendicular to the magnetic
north direction, and tilt the coil by an amount equal to
the magnetic inclination. The coil is held in the
tilted position by two brass strips that slide onto rods
on either side of the platform. The holes in the brass strips allow the angle
to be fixed at approximately five degree increments from zero to ninety
degrees.

The following websites provide geomagnetic data to allow magnetic
inclination angle to be calculated at most locations on the Earth:

Knowing the approximate inductance of a coil is
necessary, for example, to calculate the voltage induced when the current is turned off, and later, for calculating the
resonant frequency of a sample coil, given a
capacitance. The inductance of a
coil can be approximated with the following formula [20, p42-44].

It is important to know approximately the strength of the magnetic
field inside the polarization coil, so that one knows that the field
is sufficiently large to get a proton precession signal. The magnetic
field, in units of Tesla, due to a multilayered finite length solenoid
is

where x is the distance from the geometric center of the coil along
its axis, μ = 4π*10-7 Tesla*meters/Amperes, n is turns/meter
of the wire, I is the current through the solenoid in Amperes,
h is the length of the coil in meters, is the natural logarithm
( base e ), and

where r1 and r2 are the inner radius and outer radius,
respectively, of the coil in meters.

What is the minimum field strength needed
from the polarizing coil in order to get a good signal out of the sample coil? The short answer
is, the larger the better, since a stronger field produces a stronger
signal, but of course there are practical limits. For the specifications
given in this book, you should have about 75 Gauss in the geometric
center of the polarizing coil, i.e. B(0)≈0.0075 in the formula
above.

The polarizing coil form is composed of three parts,
all made of acrylic (also called plexiglass ):
the tube, and the two end pieces. The two end pieces, which are squares 6
inches (15.24 cm) on a side, need to have circular holes with a diameter of 4
1/4 inches (10.795 cm) drilled into their centers. Keep one of the discs left
over from drilling the holes. It will be used later for attaching to the
sample coil. Four small holes need to be drilled near each of the corners of
one end piece so that the polarizing coil can be attached to the tiltable
platform. Acrylic solvent can
then be used to glue the end pieces onto the tube. The acrylic and solvent can
be purchased at a plastics supplies store.

The top piece of the tiltable platform needs to have
four holes drilled into each of its four corners, so that the polarizing
coil can be mounted on it, using the nylon machine screws.

Two acrylic hinges are used to physically attach the top and bottom
pieces together. The arylic hinges are glued to the two pieces using
acrylic solvent.

Holes are drilled into the sides of the platform, opposite the hinges,
so that brass rods, can be inserted. The holes should be just wide
enough for the rods to snuggly slide in, and should be about 1/2 inch
(1.27 cm) deep. Cut 4 rods about 1 inch (2.54 cm) long, and slide
them into the holes with a dab of glue so that they won’t ever come
out, and so that they protrude out about 1/2 inch. These brass rods
are for attaching the brass strips, so that the platform angle can
be adjusted. The two brass strips need to have holes drilled into
them so that the platform angle can be adjusted in 5 degree increments.